Tuesday, May 10, 2011

Root Cause Failure Analysis: Part 2

Initially, the difference in densities of the reactants was believed to be the cause of the squishing effect. To test this theory, CFD-ACE+ software was used to model flow conditions before, at, and after the channel junction. The results appear below in the figure.

Figure 2: Reactant concentration as a function of distance into channel

In the figure above, the pink color represents concentrated soy oil, while the blue color indicates concentrated methanol. A cursory glance at the channel junction confirms that the squishing effect indeed takes place. However, the simulation shows that mixing should occur after about ten unit cells into the mixing channel. This is indicated by a gradual color change from pink and blue to orange. Stronger hues of orange seen further along in the channel indicate more thorough mixing. Therefore, the simulation confirmed suspicions of squishing but also proved it had little impact on the group’s failure to synthesize biodiesel. Furthermore, it was determined that the difference in fluid viscosities was the driving force behind the squishing effect, as opposed to the difference in densities, as originally theorized.

Next, non-ideal temperature was investigated as a possible failure mode. As a general rule of thumb, most chemical reactions increase in rate as temperature increases. In light of this, relatively high reaction temperatures in laboratory experimentations were sought. Tests conducted in the first semester demonstrated a hesitance to increase reaction temperature beyond 60°C due to the boiling point of methanol, which is 62.5°C. Second semester tests utilized reaction temperatures of 70°C and 80°C. The latter temperature was especially important for two reasons. First, the rate constant of the reaction was determined experimentally at this temperature and subsequently used in the software simulation (as will be discussed in detail in the Therory for Design section). Second, full conversion of biodiesel at 80°C was achieved in batch testing at the University of Tennessee at Chattanooga (UTC). Thin Layer Chromatography (TLC) analysis indicated no presence of biodiesel in the product at both elevated temperatures. To ensure no conversion whatsoever, a test was run at 80°C and its product sent in two samples to UTC for nuclear magnetic resonance (NMR) testing. These tests also confirmed that no conversion had taken place. Additional information on TLC analysis appears in Appendix C.

Finally, the nickel oxide (NiO) catalyst was investigated as a potential cause of failure. During experimentation, the group had to operate using several critical assumptions. First, it was assumed that the catalyst was reusable, that is, no nickel oxide was consumed in any reaction in the mixing channel. This was a reasonable assumption to make considering catalysts do not participate in their reactions by definition. Second, the group assumed that the catalyst was bonded permanently to the mixing channel walls and was not removed during flow. Finally, it was assumed that the catalyst was indeed active during flow through the channel. The basis for this was a study done by UTC in which nickel oxide was shown to be a viable catalyst for biodiesel synthesis. Comparing the forms of nickel oxide introduced to the reactants in UTC’s batch testing and in the group’s microreactor gave some critical insight in that the former method utilized nickel oxide nanopowder, while the latter used a sputtered catalyst. It was theorized that the force of the nickel oxide against the channel walls during sputtering had deactivated the catalyst, thereby destroying its catalytic properties.

After ruling out bad mixing and non-ideal temperature as possible failure modes, it was determined that deactivation of the NiO catalyst was the root cause of failure.